RNA interference (RNAi) is considered as a naturally occurring adaptive defence in at least some organisms against viruses and the production of aberrant transcripts, such as those produced by transposon mobility (Bosher and Labouesse, 2000; Waterhouse et al., 2001).
The actual process by which dsRNA mediates target RNA degradation is not fully understood, but the cellular machinery involved is gradually being identified. Full-length dsRNAs have been observed to be progressively degraded into ˜21-nucleotide dsRNAs, by an enzyme called Dicer-1 (Elbashir et al., 2001). It is believed that the Dicer-1 proteins, along with their associated 21-mer dsRNA, seek single stranded RNAs with sequence identity, and promote the cleavage of single stranded RNA targets (Waterhouse et al., 2001).
The intestine of C. elegans is a simple tube constructed of only 20 cells (White, 1988). For C. elegans, dsRNA has been microinjected into the gonadal tissues of adult worms, but simpler methods have since been developed that circumvent the tedious microinjection method. Nematodes fed Escherichia coli bacteria that simultaneously express sense and antisense RNAs can acquire dsRNA. Interestingly, the ingested dsRNA can then spread from the gut to target almost all tissues with the nematode (Timmons and Fire, 1998). Alternatively, the worms can be soaked in dsRNA solutions, either with liposomes or as naked RNA (Tabara et al., 1998; Maeda et al., 2001).
Arthropod guts are comprised of a great many cell types, and are highly variable, as they have adapted to the needs of each species and their unique dietary choices. The evolutionary distance between nematodes and insects is considerable, and there is no reason to assume that while feeding dsRNA to C. elegans was successful, it would be a technique easily transferable to insects. The presence of specific barriers in insect guts, such as the peritrophic membrane, could also limit or prevent direct absorption of orally delivered dsRNA. The midgut of an arthropod is the primary site of nutrient uptake, and midgut internal environments of different arthropods can vary widely. For example, the fruit fly Drosophila melanogaster has a rather acidic midgut lumen, while many Lepidoptera (moths and butterflies) have a very hostile, highly basic midgut environment.